The present invention concerns a method for the production of metal by reduction of a feedstock comprising an oxide of a metal. As is known from the prior art, electrolytic processes may be used, for example, to reduce metal compounds or semi-metal compounds to metals, semi-metals, or partially-reduced compounds, or to reduce mixtures of metal compounds to form alloys. In order to avoid repetition, unless otherwise indicated the term metal will be used in this document to encompass all such products, such as metals, semi-metals, alloys, intermetallics. The skilled person will appreciate that the term metal may, where appropriate, also include partially reduced products.
In recent years, there has been great interest in the direct production of metal by direct reduction of a solid metal oxide feedstock. One such direct reduction process is the Cambridge FFC® electro-decomposition process, as described in WO 99/64638. In the FFC process, a solid compound, for example a metal oxide, is arranged in contact with a cathode in an electrolysis cell comprising a fused salt. A potential is applied between the cathode and an anode of the cell such that the compound is reduced. In the FFC process, the potential that produces the solid compound is lower than a deposition potential for a cation from the fused salt.
Other reduction processes for reducing feedstock in the form of a cathodically connected solid metal compound have been proposed, such as the Polar® process described in WO 03/076690 and the process described in WO 03/048399.
Typical implementations of direct reduction processes conventionally use carbon-based anode materials. During the reduction process the carbon-based anode materials are consumed and the anodic product is an oxide of carbon, for example gaseous carbon monoxide or carbon dioxide. The presence of carbon in the process leads to a number of issues that reduce the efficiency of the process and lead to contamination of the metal produced by reduction at the cathode. For many products it may be desirable to eliminate carbon from the system altogether.
Numerous attempts have been made to identify so-called inert anodes that are not consumed during electrolysis and evolve oxygen gas as an anodic product. Of conventional, readily-available materials, tin oxide has shown some limited success. A more exotic oxygen-evolving anode material based on calcium ruthenate has been proposed, but the material has limited mechanical strength, suffers from degradation during handling, and is expensive.
Platinum has been used as an anode in LiCl-based salts for the reduction of uranium oxide and other metal oxides, but the process conditions need to be very carefully controlled to avoid degradation of the anode and this too is expensive. Platinum anodes are not an economically viable solution for an industrial scale metal production process.
While an oxygen-evolving anode for use in the FFC process may be desirable, the actual implementation of a commercially viable material appears to be difficult to achieve. Furthermore, additional engineering difficulties may be created in the use of an oxygen-evolving anode, due to the highly corrosive nature of oxygen at the high temperatures involved in direct electrolytic reduction processes.
An alternative anode system is proposed in WO 02/083993 in which the anode in an electrolysis cell was formed from molten silver or molten copper. In the method disclosed in WO 02/083993 oxygen removed from a metal oxide at the cathode is transported through the electrolyte and dissolves in the metal anode. The dissolved oxygen is then continuously removed by locally reducing oxygen partial pressure over a portion of the metal anode. This alternative anode system has limited use. The removal of oxygen is dependent on the rate at which the oxygen can diffuse into the molten silver or copper anode material. Furthermore, the rate is also dependent on the continuous removal of oxygen by locally reducing partial pressure over a portion of the anode. Thus, this process does not appear to be a commercially viable method of producing metal.